1. Field of the Invention
The present invention relates to a terminal lead, a method of producing the terminal lead, and an electrochemistry device (e.g., a lithium ion secondary battery, an electric double-layer capacitor) equipped with the terminal lead.
2. Background Art
For example, in a lithium ion secondary battery as an electrochemistry device, a battery element including electrodes (current collectors) and electrolyte (electrolyte solution) are enclosed in an exterior casing. The opening edge portions of the exterior casing are welded together by heat sealing, so that the battery element is sealed in the exterior casing (see, e.g., Patent Document 1).
An outer film forming the exterior casing is constituted by a plurality of layers including an outermost layer made of polyethylene terephthalate (PET) or nylon and an innermost layer which comes into contact with electrolyte made of heat adhesive resin. An intermediate layer of the outer film is constituted by a metallic foil, such as, e.g., an aluminum foil, a SUS foil (stainless steel foil), etc., to prevent invasion of water from the outside and evaporation of the electrolyte from the inside. The outermost layer of the outer film is designed to protect the metallic foil of the intermediate layer and provides protection against possible external force such as stabbing.
A terminal lead of a battery equipped with a battery element accommodated in an exterior casing is generally equipped with a plate-shaped metallic substrate as a base material. The terminal lead is integrally provided with an inner end portion arranged inwardly of the exterior casing and an outer end portion arranged outwardly of the exterior casing and extended outwardly from the seal portion of the exterior casing. A portion of the terminal lead corresponding to the seal portion of the exterior casing is welded to the outer film with a heat adhesive resin of the innermost layer of the outer film by heat sealing. In some cases, however, the terminal lead may penetrate the innermost layer of the outer film to be brought into contact with the metallic foil as an intermediate layer, resulting in electric short.
To secure electrical insulation between the terminal lead and the metallic foil of the intermediate layer, an insulating resin film is arranged at the portion of the terminal lead corresponding to the seal portion of the exterior casing (see, e.g., Patent Documents 2 to 6).
The adhesion between the terminal lead and the insulating resin film decreases as time passes due to influence of electrolyte. To prevent the problem, in some conventional terminal leads, a chitosan layer containing chitosan or chitosan derivative is formed on the entire surface of the metallic substrate by a coating method as a surface coating layer (see, e.g., Patent Documents 4 and 6).